The present invention relates to improvements in lighting control apparatus of the type adapted to dim a plurality of different types of light sources (e.g. incandescent, fluorescent, neon, etc.) and to provide a visual indication of the instantaneous level of dimming, for example, by the number of lights illuminated in a linear array of LED's (light-emitting diodes) or the position of a potentiometer slider (used to set the dimming level) in a linear track.
Commonly assigned U.S. Pat. Nos. 4,575,660; 4,924,151; and 5,191,265 disclose various lighting control systems in which groups of lights, defining a lighting zone, are varied in brightness to produce several different scenes of illumination. The level of brightness of the lights constituting each lighting group is displayed to the user by either the number of LED's illuminated in a linear array of LED's, or the position of a potentiometer slider in a linear track. For example, if the number of LED's in the array is ten, illuminating six LED's would indicate that the lights in a particular zone are operating at 60% of maximum brightness. Similarly, if the position of the dimmer actuator (slider) is set at about three-tenths of its maximum allowed movement, the perceived light level will be at about 30% of maximum. So long as all light sources are of the same type, e.g. all incandescent, the light level indicators of the above lighting control systems accurately reflect the instantaneous lighting levels of the different lighting zones. But, when the light sources differ from zone-to-zone, the accuracy of the light level display is compromised. Moreover, a given change in dimmer setting will not produce the same change in light output form of the different sources.
To understand the problem alluded to above, one must understand that such dimmers operate by a phase control scheme in which the power applied to a light source from an AC power source is interrupted each half-cycle by a predetermined phase angle, the larger the angle, the lower the power applied to the source and, hence, the lower its brightness. The power interruption may be at the beginning of each half-cycle, in the middle or at the end (as in the case of reverse phase control). The maximum and minimum allowable phase angles (which determine the minimum and maximum brightness, respectively, of a given light source) are characteristics of the particular light source. In the case of an incandescent lamp, the phase angle may be theoretically varied from zero to 180 degrees; however, for a variety of reasons, it is usually desirable to operate at phase angles between about 40 and 160 degrees. In the case of fluorescent lamps, the range of allowable phase angles is narrower, owing to the need to maintain a certain current in the lamp to avoid flicker or extinction of the gas plasma. A typical operating range of phase angles for fluorescent lamps is between about 50 and 120 degrees. Other types of lamps, notably neon, have a different and even narrower range of acceptable phase angles for maximum and minimum light output, a typical range for neon lamps being between about 70 and 130 degrees. It is these different ranges of acceptable phase angles that give rise to the aforementioned problems of the above lighting control systems. If, for example, the potentiometer slide is normalized for an incandescent source, movement of the slider from one end of its track to the other will cause the phase angle to change by a total of 120 degrees. If, instead of an incandescent source, a fluorescent source is and in the same zone, the first 30% of the slider movement will be dead travel, and no change in light output will occur until the phase angle reaches 120 degrees. The same effect occurs, to a lesser extent, at the upper end of the slider movement. Similarly, if the aforementioned ten LED display is set up for incandescent lamps and other types of lamp (e.g. fluorescent) are used, the bottom three LED's will be energized, indicating 30% light level when, in fact, the fluorescent source will not yet have begun to radiate energy.